1. Field of the Invention
The invention provides fuel cells, in particular PEM fuel cells in which a solid polymer is used as electrolyte.
2. Description of Prior Art
Fuel cells convert a fuel and an oxidising agent which are spatially separated from each other at two electrodes into electricity, heat and water. Hydrogen or a hydrogen-rich gas may be used as the fuel and oxygen or air as the oxidising agent. The process of energy conversion in the fuel cell is characterised by particularly high efficiency. For this reason, fuel cells in combination with electric motors are becoming more and more important as an alternative to traditional internal combustion engines.
The so-called polymer electrolyte fuel cell (PEM fuel cell) is suitable for use as an energy converter in motor vehicles because of its compact structure, its power density and its high efficiency.
The PEM fuel cell consists of a stacked arrangement (“stack”) of membrane electrode assemblies (MEAs), between which are arranged bipolar plates for supplying gas and conducting electrical current. A membrane electrode assembly consists of a polymer electrolyte membrane, to both sides of which are applied reaction layers and gas distributor layers. One of the reaction layers is designed as an anode for the oxidation of hydrogen and the second reaction layer is designed as a cathode for the reduction of oxygen. The arrangement of reaction layer and gas distributor layer is called an electrode for the membrane electrode assembly in the context of this invention. The gas distributor layers usually consist of carbon fibre paper or a non-woven carbon cloth and facilitate good access by the reaction gases to the reaction layers and effective removal of the cell current. The reaction layers for anodes and cathodes contain so-called electrocatalysts which catalytically support the particular reaction (oxidation of hydrogen or reduction of oxygen). Metals from the platinum group in the Periodic Table of Elements are preferably used as the catalytically active components. In the majority of cases, so-called supported catalysts, in which the catalytically active platinum group metal has been applied in highly dispersed form to the surface of a conductive support material, are used. The average crystallite size of the platinum group metals is between about 1 and about 10 nm. Finely divided carbon blacks have proved useful as support materials.
The polymer electrolyte membrane consists of proton-conducting polymer materials. These materials are also called ionomers for short in the following. A tetrafluorethylene/fluorovinylether copolymer with acid functions, in particular sulfonic acid groups, is preferably used. Such a material is sold, for example, under the tradename Nafion® by E.I. DuPont. However, other, in particular fluorine-free, ionomer materials such as sulfonated polyetherketones or arylketones or polybenzimidazoles may also be used.
U.S. Pat. No. 4,229,490 discloses a process for producing a fuel cell electrode. This process comprises hydrophobizing a carbon fibre paper and then coating with a graphite/platinum black/PTFE mixture and sintering. Fuel cell electrodes produced in this way have a high platinum load and do not contain a proton-conducting polymer. Thus only a small part of the platinum used is contacted in such a way that it can take part in the electrolytic process.
U.S. Pat. No. 4,876,115 describes a process for treating a porous gas diffusion electrode which has a catalyst load of less than 0.5 mg/cm2 on carbon particles. The electrode is impregnated with a solution of a proton-conducting material. This coats the surfaces of the carbon particles with the proton-conducting material.
U.S. Pat. No. 5,234,777 discloses a membrane electrode assembly which consists of a polymer electrolyte membrane and a layer formulated from a platinum supported catalyst and an ionomer. This layer is characterized in that it is less than 10 μm thick and the platinum supported catalyst is dispersed uniformly in the proton-conducting ionomer. The platinum load on the electrode is less than 0.35 mg/cm2. The electrode layers are in contact with the polymer electrolyte membrane.
Various processes are described for producing membrane electrode assemblies according to U.S. Pat. No. 5,234,777. In one embodiment, the Pt/C supported catalyst is dispersed in an alcoholic solution of the ionomer. This dispersion, also called an ink, is applied to a PTFE film release blank (PTFE: polytetrafluorethylene), dried and laminated onto the opposite faces of a polymer electrolyte membrane by hot pressing.
In another embodiment, the polymer electrolyte membrane is coated directly with an ink of a Pt/C supported catalyst and a solution of an ionomer. The applied layer is dried at a temperature of at least 150° C.
The reaction layers according to U.S. Pat. No. 5,234,777 are characterized by a homogeneous distribution of catalyst in the ionomer. As a result of hot pressing, dense and pore-free layers with a thickness of less than 10 μm, preferably 5 μm and with platinum loads of less than 0.35 mg Pt/cm2 are produced. In the case of membrane electrode assemblies according to U.S. Pat. No. 5,234,777, due to the dense, pore-free reaction layer, access by the reaction gases to the catalyst is restricted. This has a negative effect on the electrochemical performance of the PEM cell, in particular when operating with dilute gases such as air or reformate gas. The possible use of air and reformate gas instead of oxygen and hydrogen, however, is an important prerequisite for the economically viable use of fuel cells in motor vehicles.
A further disadvantage of the process described in U.S. Pat. No. 5,234,777 is the high drying temperature of at least 150° C. Under these conditions, solvent vapours in contact with the catalyst layers can ignite and destroy the membrane electrode assembly.
DE 196 02 629 A1 discloses a process for producing a membrane electrode assembly in which a noble metal catalyst on a carbon support is used, on which the ionomer is adsorbed as a colloid. To achieve this, a colloidal solution of the ionomer is prepared in a suitable organic solvent and the supported catalyst is treated therewith. The supported catalyst coated with colloid is processed to form an ink and an electrode is prepared therewith which is compression moulded with the polymer electrolyte membrane.
Membrane electrode assemblies produced according to DE 196 02 629 A1, however, do not exhibit improved access by the reaction gases to the catalyst. Furthermore, it is difficult to achieve defined and reproducible distribution of the ionomer in colloidal form on the supported catalyst. The stability of the colloidal ionomer is limited. Transfer of the process to mass-production is thus possible to only a limited extent.
EP 0 797 265 A1 describes a membrane electrode assembly for PEM fuel cells with a high total porosity and improved electrochemical performance. The high porosity is achieved by using pore-producers in combination with a specific spray process. The process has the disadvantage that the pore-producers lead to contamination and additional steps are required in order to remove the pore-producers from the membrane electrode assembly.
For wide commercial use of PEM fuel cells in motor vehicles, further improvement in the electrochemical cell performance and a clear reduction in the system costs is required. This is a prerequisite for electrical drives using power supplied by fuel cells being able to compete successfully with traditional internal combustion engines.
In order to increase the efficiency, the performance of fuel cells when operated under a part load, that is to say at low current density, must be further increased. In order to achieve this, the structure of the reaction layers containing the electrocatalyst has to be further improved.